The present invention relates to a prediction system which permits high efficiency coding of a picture signal as of commercial television or video telephony through utilization of high correlation between picture elements in a picture.
The existing television is made up of a picture called a frame which is sent at a rate of 30 per second, and each frame consists of two successive fields because of interlaced scanning being carried out every other scanning lines. An element constituting the picture is referred to as a "picture element" but, in this specification, one sample obtained by sampling will be called a picture element in view of digital processing. Accordingly, in this case, the position of each picture element in the picture depends upon the sampling frequency for digitizing a signal.
Therefore, in order to achieve stable coding efficiency by employing mainly the inter-frame prediction coding of the inter-field prediction coding for the still picture and mainly the intra-field prediction coding or the inter-field prediction coding for the moving picture, two representative methods such as described below have been employed in conventional arts.
A first one of them is a system in which a field of a television picture is divided into blocks, each containing n.multidot.m picture elements; inter-frame prediction, inter-field prediction and intra-field prediction are effected for each block; prediction values produced by the respective predictions are compared with one another; and that one of prediction circuit which produces the prediction value closest to the true value is utilized as a prediction circuit for a block of an input signal. With this method, it is necessary to transmit to the receiving side over head information indicating the prediction circuit adopted for each block of the input signal, in addition to prediction error information. Accordingly, when to reduce the block size for further fine and adaptive prediction coding, the ratio of the over head information to the transmission rate increases, making it impossible to enhance the coding efficiency.
Another method is a system in which a prediction circuit, which provides a prediction value closest to one pre-coded picture element value, for example, the value of a picture element lying immediately before the current picture element to be coded, is used as a prediction circuit for the current picture element. With this method, since there is no need of transmitting the over head information to the receiving side, fine and adaptive prediction coding is possible; however, the adopted prediction circuit does not always provide the value closest to the current picture element value, and hence high coding efficiency cannot be expected.